Field of the Invention
The present invention relates to a structuring method, in particular a method for structuring layers which can only be etched with difficulty by plasma-chemical or by dry-chemical methods or cannot be etched at all. Such layers are generally composed of inert metals, ferroelectric materials and dielectric materials with a high relative dielectric constant.
During the development of highly integrated memory modules, such as DRAMs and FRAMs, for example, the cell capacity during the progressive miniaturization should be retained or even improved. In order to achieve this object, ever thinner dielectric layers and folded capacitor electrodes (trench cell, stack cell) are used. Instead of using conventional silicon oxide, new materials, in particular paraelectrics and ferroelectrics, are used between the capacitor electrodes of a memory cell. For example, barium strontium titanate (BST, (Ba,Sr)TiO.sub.3), lead zirconate titanate (PZT, Pb(Zr,Ti)O.sub.3) and/or lanthanum-doped lead zirconate titanate or strontium bismuth tantalate (SBT, SrBi.sub.2 Ta.sub.2 O.sub.9) are used for the capacitors of the memory cells in the DRAMs and/or FRAMs.
There, the materials are usually deposited on existing electrodes (bottom electrodes). Processing takes place at high temperatures, with the result that the materials from which the capacitor electrodes are normally composed of, for example doped polysilicon, are easily oxidized and lose their electrically conductive properties, which can lead to memory cell failure.
Because of their good resistance to oxidation and/or because of the formation of electrically conductive oxides, 4d and 5d transition metals, in particular platinum metals (Ru, Rh, Pd, Os, Ir, Pt) and in particular platinum, and also rhenium are promising candidates which could replace doped polysilicon as the electrode material in the abovementioned memory cells.
The progressive miniaturization of the components also has the result that replacement materials for the aluminium commonly used for the conductor tracks also became necessary. The replacement material here should have a lower specific resistance and a lower degree of electromigration than aluminium. A promising candidate here is copper. Furthermore, the development of magnetic "Random Access Memories" (MRAMs) requires the integration of magnetic layers (for example Fe, Co, Ni or permalloy) into microelectronic circuits.
In order to be able to construct an integrated circuit from the above-mentioned materials, which have hitherto not been widespread in semiconductor technology, thin layers of these materials must be structured.
The structuring of the previously used materials is carried out as a rule by so-called plasma-supported anisotropic etching methods. Here, physical-chemical methods are usually applied in which gas mixtures composed of one or more reactive gases, such as oxygen, chlorine, bromine, hydrogen chloride, hydrogen bromide and/or halogenated hydrocarbons and of inert gases (for example Ar, He). The gas mixtures are as a rule excited in an alternating electromagnetic field at low pressures.
The principles of the method of operation of an etching chamber, are illustrated by example of a parallel plate reactor. A gas mixture, for example Ar and Cl.sub.2, is fed to a reactor chamber via a gas inlet and pumped out again through a gas outlet. A lower plate of the parallel plate reactor is connected to a high-frequency source by a capacitor and serves as a substrate holder. By applying a high-frequency alternating electric field to an upper plate and the lower plate of the parallel-plate reactor, the gas mixture is converted into a plasma. Since the mobility of the electrons is greater than that of the gas cations, the upper and lower plates become negatively charged with respect to the plasma. For this reason, both plates exert a high force of attraction on the positively charged gas cations, with the result that they are subjected to a permanent bombardment by the ions, for example Ar.sup.+. Since the gas pressure is kept low, typically 0.1-10 Pa, there is only a low degree of scattering of the ions with respect to one another and to the neutral particles, and the ions strike virtually perpendicular against the surface of a substrate which is secured to the lower plate of the parallel-plate reactor. This permits an image of a mask to be well formed on the underlying layer of the substrate to be etched.
Usually, photoresists are used as mask materials since they can be structured relatively simply by an exposure step and a development step.
The physical part of the etching is brought about by the impetus and kinetic energy of the incident ions (for example Cl.sub.2 +, Ar+). In addition, chemical reactions between the substrate and the reactive gas particles (ions, molecules, atoms, radicals) are initiated or promoted (chemical part of the etching) thereby, accompanied by the formation of volatile reaction products. The chemical reactions between the substrate particles and the gas particles are responsible for high etching selectivities of the etching process.
Unfortunately, it has become apparent that the above-mentioned materials, which have newly been brought into use in integrated circuits, are among those materials which cannot be etched or can be etched chemically only with difficulty and in which the etching erosion is based, even when "reactive" gases are used, predominantly or almost exclusively on the physical component of the etching.
Because the chemical component of the etching is small or absent, the etching erosion of the layer to be structured is of the same order of magnitude as the etching erosion of the mask and/or of the substrate (etching barrier layer), i.e. the etching selectivity with respect to the etching mask and/or substrate is generally small (between approximately 0.3 and 3.0). The result of this is that due to the erosion of masks with inclined edges and the unavoidable formation of facets (beveling, tapering) on the masks only a low degree of dimensional accuracy of the structuring can be ensured. The faceting thus restricts the smallest structure sizes which can be achieved during the structuring and the achievable steepness of the profile edges on the layers to be structured.
At the same time, the faceting on the masks, and thus also the faceting of the layers to be structured, is greater the greater the proportion of reactive gases (in particular chlorine) in the gas mixture which is used during the plasma-chemical etching method. Correspondingly, gas mixtures which do not have any proportion of reactive gases, for example pure argon plasmas, can be used to produce the steepest profile edges on the layers to be structured.
In addition to the aforesaid faceting of the layers to be structured, undesired redepositions of the material of the layer to be structured may also occur during the structuring. Redeposition, as used herein in the context of physical plasma etching, corresponds to the generally accepted usage of the term in the semiconductor industry. See, i.e., U.S. Pat. No. 6,027,860 to McClure et al. The redepositions occur, for example, on the side walls of the resist mask and to date it has only been possible to remove them incompletely, if at all, in the subsequent post-treatment steps. Unfortunately the occurrence of the redepositions becomes all the more pronounced the smaller the proportion of reactive gases in the gas mixture which is used during the plasma-chemical etching method. Correspondingly, the process control has hitherto usually been limited to small argon proportions, for example in a chlorine/argon plasma. However, the increased proportion of chlorine in the etching gas mixture leads in turn to an increased formation of facets in the masks.
In the case of platinum etching with a resist mask, the use of reactive gases such as chlorine or HBr results in intermediate redepositions forming which disappear again during the further course of the etching. These structures also lead to CD widening and to flat platinum edges. They have been found to be the greatest disadvantage of a process which uses both chlorine and a resist mask.
If, instead of a resist mask, a so-called "hard mask" is used to structure the layers, many of the aforesaid difficulties can be significantly reduced. However, the structuring of a "hard mask" requires additional process steps which make the entire process more expensive.